> The cat sat on the mat after the party.
The rule is that this doesn't require a comma because it comes at the end. Optional comma, if you move it to the front.
That seems right, a prepositional phrase at the front can get a comma, and sometimes also one in the middle.
It is a tale
told by an idiot, full of sound and fury,
signifying nothing.
It is a tale
told by an idiot, full of sound and fury,
signifying nothing.
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